When drilling oil or gas wells, a wellbore is formed. After drilling, the drill string and bit are removed and the remaining wellbore is lined with a metal casing. A generally annular area is formed between the outside surface of the metal casing and the surrounding formations.
A cementing operation is typically conducted to fill the area between the metal casing and the surrounding formation with concrete. The combination of concrete and metal casing strengthens the wellbore.
Later, perforations are usually made in the metal casing and concrete using a perforating gun assembly that is generally comprised of a steel carrier, and a charge tube inside of the carrier with shaped charges positioned in the charge tube. The perforating gun is lowered into the wellbore and is typically connected to an electric wireline or other conveyance device until it is at a predetermined position. Then a signal actuates a firing head of the gun, which detonates the shaped charges in the gun. The explosion of the shaped charges perforates the metal casing and concrete to allow fluids to flow from the formation into the wellbore.